1. Field of the Invention
The present invention relates generally to offset printing presses and more particularly to a tubular printing blanket for an offset lithographic press.
2. Background Information
U.S. Pat. No. 5,429,048 to Gaffney et al purports to disclose an offset lithographic printing press in which a web of material to be printed passes through a series of print units. Each of the print units has an upper plate cylinder, an upper blanket cylinder, a lower blanket cylinder and a lower plate cylinder. Printing plates containing images may be fastened to the plate cylinders and gapless tubular-shaped blankets may be fastened to the blanket cylinders. During a printing operation, the web passes between the upper blanket and the lower blanket. The tubular-shaped blanket of the printing press includes an outer print layer of incompressible material, a second layer of compressible material under the print layer and a third layer of incompressible material attached to a nickel sleeve. The patent mentions at column 12, line 19 et seq. that a deflectable fabric or inextendable layer could be provided between or inside each of the layers.
U.S. Pat. Nos. 5,304,267 and 5,323,702 purport to disclose tubular printing blankets having an outer print layer of elastomeric material, an inextensible layer under the print layer, and a compressible layer under the inextensible layer which is connected to a nickel sleeve. The inextensible layer is made of a seamless tubular body of elastomeric material and a longitudinally inextensible thread within the tubular body 100. The patent also discloses that the inextensible layer could be formed of a seamless tubular body of material such as a urethane copolymer, thermosetting polymers and rubbers. This material has a modulus of elasticity in the range of 2500-10,000 psi. Alternatively, the patent discloses an embodiment in which a continuous piece of plastic film extends in a spiral through an elastomeric material of an inextensible layer. The plastic film is not joined or merged together and does not form an isotropic layer.
U.S. Pat. No. 5,352,507 purports to disclose a sleeve-like printing blanket. Underneath a printing layer is an elastomeric layer having reinforcing fibers therein. The elastomeric layer may be an elastomeric-impregnated nonwoven mat, of, for example, spunlaced aramid fibers or spunbonded nonwoven polyester. Holes in the nonwoven are filled with elastomeric material in a complex impregranation step. The reinforced elastomeric layer then is placed on the blanket either by spiral wrapping or by wrapping a sheet of material to abut, and then curing so that the elastomeric material tends to merge or meld. As best understood, only the elastomeric material melds together while the material of the mat does not meld or merge.
The reinforcing layers of the blankets discussed above have been concerned primarily with providing stability in the radial direction. However, the tubular printing blankets disclosed above suffer from the problem that the reinforcing layers underneath the printing layers lack stiffness and strength in the cross-web or axial direction. The lack of stiffness and strength in the cross-web direction can lead to a relatively short lifespan, as circumferential cracks can form which can eventually lead to delamination. The reinforcing layers discussed above also are formed directly over the compressible layer, which can lead to difficult curing requirements which can be expensive and may damage, for example, the compressible layer.
U.S. Pat. Nos. 4,981,750 and 4,042,743 purport to disclose flat blankets which have a reinforcing layer underneath the printing layer. These blankets however must be clamped in a clamping mechanism and cannot be used as tubular blankets.
An object of the present invention is to improve the durability of tubular blankets. Another additional or alternative object is to provide for a simple method of manufacturing tubular blankets.
The present invention provides a tubular printing blanket including a tubular print layer, a tubular isotropic thermoplastic reinforcing layer underneath the print layer, a compressible layer underneath the reinforcing layer, and a sleeve for supporting, directly or indirectly, the compressible layer.
The tubular isotropic thermoplastic reinforcing layer provides axial stability due to its isotropic nature and the thermoplastic material. xe2x80x9cThermoplasticxe2x80x9d as defined herein includes thermosetting materials.
Preferably, the tubular printing blanket includes a build-up layer between the sleeve and the compressible layer. The build-up layer preferably is made of a hard rubber material.
Preferably, the reinforcing layer is made of polyester, most preferably MYLAR or MELENEX from the DuPont Corporation, and is preformed in a tubular shape so as to be isotropic. The MYLAR may be approximately 0.008 inches thick, with an inner diameter when relaxed of 7.0504 inches, for example. Preferably, the reinforcing layer is made of a homogeneous thermoplastic film. Thus isotropic tubular reinforcing layer preferably is made solely of thermoplastic sheet material.
The present invention also provides a tubular print layer, a tubular isotropic reinforcing layer underneath the print layer and having a modulus of elasticity in the range of 100,000 to 1,000,000 psi, a compressible layer underneath the reinforcing layer, and a sleeve for supporting, directly or indirectly, the compressible layer. This high modulus of elasticity aids in providing axial stability. Preferably, the modulus of elasticity is 500,000 to 1,000,000 psi.
The present invention also provides a method for manufacturing a tubular printing blanket including forming a tubular reinforcing layer and sliding the tubular reinforcing layer axially over a compressible layer. The compressible layer is supported by a sleeve. A printing layer then is added over the reinforcing layer.
Preferably the tubular reinforcing layer is made of thermoplastic, most preferably a polyester film such as MYLAR. Preferably, the tubular reinforcing layer is isotropic.
Advantageously, the method may include providing compressed air to an inner surface of the reinforcing layer during the sliding step. Once the reinforcing layer is slid over the compressible layer the compressed air is no longer provided. The reinforcing layer relaxes to form a fit with the compressible layer. Preferably, the tubular reinforcing layer is first slid over a mandrel cap which has a diameter larger than the sleeve.
A device for manufacturing a tubular printing blanket is also provided. A first build cylinder has an outer diameter. A mandrel cap can be attached to one end of the build cylinder, the mandrel cap having an outer diameter larger than the build cylinder outer diameter. The mandrel cap may include air holes at its outer diameter for providing pressurized air to the inside of an isotopic reinforcing layer.
The blankets of the present invention can attain a longer lifespan because of improved axial strength due to the isotropic reinforcing layer. The manufacture of the blanket using the tubular reinforcing layer also removes the need for a curing step on the build cylinder in forming the reinforcing layer. The pre-forming of the reinforcing layer in mass-production ahead of time can reduce manufacturing costs.